An internal combustion engine disclosed in Patent Literature 1 (JP 2011-106377 A) comprises: an injector which has a plurality of injection holes; and a spark plug, the injector and the spark plug being provided in an upper part of a combustion chamber. In the internal combustion engine, a distance from a center position of a discharge gap of the spark plug to a center position of the injection hole which is closest to the spark plug among the plurality of injection holes is set within a specific range. In the internal combustion engine, a control for applying a high voltage to the spark plug is performed over a period from a time point after a lapse of a predetermined time from the start of a fuel injection to a time point when the fuel injection is completed.
In the above-described control, a fuel injection period of the injector overlaps with a period of applying the high voltage to the spark plug. When the fuel is injected by the injector which is supplied with the fuel in a pressurized condition, a low pressure area is formed by entraining air around the fuel spray injected from each injection hole (entrainment). Therefore, when the above-described control is performed, a discharge spark generated in the discharge gap is attracted to the low pressure area formed by the fuel spray from the injection hole closest to the spark plug. The internal combustion engine can thereby improve ignitability of an air-fuel mixture formed around the spark plug.
Patent Literature 1 further introduces activation of an exhaust gas cleaning catalyst as applications of the above described attraction action. Although not mentioned in Patent Literature 1, the exhaust gas cleaning catalyst is generally activated by changing an ignition period, which is normally set near a compression top dead center (i.e., a period of applying a high voltage to the spark plug), to a period retarded from the compression top dead center.
When the above-described control of Patent Literature 1 is applied for the general activation of the exhaust gas cleaning catalyst, the ignition period set at a retarded side from the compression top dead center overlaps with a fuel injection period to improve the ignitability of the air-fuel mixture formed around the spark plug. However, if an igniting environment is changed due to some factors and therefore is out of a desired range, a combustion state may become unstable in spite of the above-described attraction action. In combustion cycles during the control for activating the exhaust gas cleaning catalyst, when the number of combustion cycles in which such a situation occurs is increased, a combustion fluctuation between cycles becomes large, and drivability is affected.
The present application addresses the above problems, and an object of the present application is to suppress the combustion fluctuation between cycles when the control performed so that the fuel injection period of the injector overlaps with the period of applying the high voltage to the spark plug is applied for the activation of the exhaust gas cleaning catalyst.